Read the message:
READ ONLY : This Document is Lock for Editing by another user
That means just what it says: the file is in use, and until it is no longer being used you can't access it.
Probably, you app is not closing iot's files properly, and the file remains in use from a "previous incarnation" of you app, or a previous operation within your app.
But there is nothing at all we can do you help you with this: you need to find out where and why - and that means access to yoru code and file system while it's running and we have neither.
So, it's going to be up to you.
Fortunately, you have a tool available to you which will help you find out what is going on: the debugger. If you don't know how to use it then a quick Google for "Visual Studio debugger" should give you the info you need.
Put a breakpoint on the first line in the function, and run your code through the debugger. Then look at your code, and at your data and work out what should happen manually. Then single step each line checking that what you expected to happen is exactly what did. When it isn't, that's when you have a problem, and you can back-track (or run it again and look more closely) to find out why.
Sorry, but we can't do that for you - time for you to learn a new (and very, very useful) skill: debugging!